My boyfriend hangs out with his ex girlfriend?

When they dated, he became good friends with a group of people who were his ex's friends at first. They broke up and now he is with me, but he regularly hangs out with that group of friends and sees his ex girlfriend. I never go with them, so I don't know how much they interact. I'm worried if he still feels something for her? I Trust him but I can't do anything about the way I feel. I'm not comfortable with this at all. What do I do?